1. Field of the Invention
The present invention relates to the field of wireless communications.
2. Description of Related Art
In wireless communications networks that use time division multiple access (TDMA), radio interface channels are defined as time slots on a given frequency. A particular channel uses the same repeating time slot, resulting in mutual orthogonality between different channels in time. The duration between when a given time slot repeats is called a frame, and multiple frames are typically organized into a larger logical structure called a multiframe. Multiframe size may vary depending on the type of information (i.e. “logical channel”) the multiframe is designed to carry. As a rule, however, all time slots on a given frequency have the same type of frame and the same type of multiframe, such that only different frequencies may have different multiframe structures.
A wireless communications network that operates in accordance with the Global System for Mobile telecommunications (GSM) is an exemplary network in which different multiframe structures are provided on different frequencies. As is well known, a GSM network uses a combination of frequency division multiple access (FDMA), whereby available frequency spectrum is divided into separate frequency carriers, and TDMA, typically having eight time slots per frame, to share available frequency spectrum resources among all users. To schedule the timing of traffic and control transmissions (“bursts”), each group of eight time slots forms a frame, and frames are grouped together to form multiframes, such that the duration between repetitions of a given frame is one multiframe. Two basic types of multiframe structures are used in GSM, having 26 and 51 frames respectively. The 26 frame multiframe structure (“26 multiframe”) primarily contains voice traffic channels and the 51 frame multiframe structure “51 multiframe”) is used for control channels, including broadcast control channels that are used for base station identification and frequency allocations and common control channels that are used during call origination and call paging. The 26 multiframe uses different frequencies than the 51 multiframe. In this way, all time slots on a given frequency have the same type of frame/multiframe structure.
By using 26 frames per multiframe for traffic and 51 frames per multiframe for control, scheduling of traffic and control bursts is inherently de-synchronized because time slot 1 of frame 1 for both multiframe structures will only occur at the same time every 26×51 frames. This de-synchronization ensures that all mobile units have adequate opportunity to hear broadcast bursts being transmitted by network base stations and allows mobile units to make interference measurements for neighboring cells regardless of when the mobile unit is scheduled to transmit and receive traffic. GSM networks support packet data services, using a standard known as General Packet Radio Service (GPRS), by overlaying a packet-based air interface on the existing circuit switched GSM network, thereby giving the user an option to use a packet-based data service or circuit-switched voice service. The packet-based air interface of GPRS uses a 52 multiframe for traffic channels, i.e., a structure that is merely two consecutive 26 multiframes.
The U.S. wireless communications community has begun evolving the IS-136 TDMA standard toward a third-generation (3G) wireless network that supports high-speed packet data services such as Internet/intranet access and other multimedia applications. To provide such packet data services, the UWCC (Universal Wireless Communication Consortium) has chosen to adopt a variation of GSM GPRS technology, which is being called EDGE-COMPACT. Due to limits on the frequency spectrum available in the U.S. for launching packet data service, however, the {fraction (1/9)} or {fraction (1/12)} frequency reuse patterns (i.e., reuse patterns where the same frequency is used by every ninth or twelfth network cell or sector) that are used to avoid interference in the typical GSM network must be simulated in EDGE-COMPACT by accurately synchronizing base stations and “time-grouping” cells or sectors to reduce interference between control bursts from proximate base stations that are transmitting on the same frequency. More specifically, EDGE-COMPACT requires that each cell or sector be designated to one of three (or four) time-groups to protect control bursts, such that when a base station for a cell or sector assigned to a first time-group transmits control bursts base stations assigned to cells or sectors in other time-groups are idle. Thus, by appropriate synchronization of base stations, three (or four) mutually orthogonal physical carriers can be achieved for each frequency reuse is used only for control channels, while the traffic channels remain in a ⅓ frequency reuse pattern (for three frequency carriers).
Unlike GPRS, EDGE-COMPACT does not have the underlying 51 multiframe of GSM that is used for control channels, and, due to the limited frequency spectrum available for implementation, has been designed to incorporate both broadcast control channels and common control channels within a single multiframe structure. For example, the 52 multiframe of EDGE-COMPACT schedules reserves frames in each multiframe for broadcast control channels such as a synchronization channel and a frequency correction channel. Since EDGE-COMPACT does not have the 51 multiframe structure of GSM, it does not support circuit-switched GSM service. Furthermore, because only a single 52 multiframe is used to schedule all transmissions in EDGE-COMPACT, it may be difficult for mobiles to measure interference power of neighboring cells, which is important for handover (for circuit-switched users) and cell-reselection (for packet-switched users), thereby negatively affecting overall network performance.